Multiple channel transmission control



April 24, 1934.- Mc| NICOLSON 1,956,397

MULTIPLE CHANNEL TRANSMISSION CONTROL Filed Feb. 21, 1931 2 Sheets-Sheet 1 Frequency INVENTOR Alexander M Lean NilZEllSDl] af i /12% ATTORNEY April 24, 1934. A; McL, o s0 1,956,397

MULTIPLE CHANNEL TRANSMISSION CONTROL Filed Feb. 21, 1931 2 Sheets-Sheet 2 ATTORNEY Patented Apr. 24, 1934 STATES PATENT: OFFICE MULTIPLE CHANNEL TRANSMISSION CONTROL Application FebruaryZl, 1931, Serial No. 517,388

8 Claims.

Thisinvention relates to electrical transmission systems, and particularly to transmission systems with which a plurality of signals are transmitted simultaneously over common apparatus.

An object of the invention is to transmit a plurality of signals simultaneously over common transmission apparatus.

Another object of the invention is the transmission of a plurality of signals in a common circuit, segregation of the signals being obtained with thermionic tubes operating at mutually exclusive intervals.

A further object of the invention is to obtain the separation of a plurality of signals in a common circuit by thermionic vacuum tube devices havin'g biases of the same value at mutually exclusive intervals.

The art of multiplex signaling has been ac- '20 complished in the past by the use of individual physical wire circuits, by polarized impulses, by the use of carrier frequencies, a carrier being allotted to each channel, by time division with the impulse method as disclosed in my copending .25 application Ser. No. 460,806, filed June 13, 1930,

and by a filter separation system disclosed in my copending application Ser. No. 510,860, filed January 24, 1931.

The present invention contemplates the use of solely thermionic means for separating the signals or signal channels employing a cyclic varying potential which correspondingly changes the biasing potentials of control valve units, the tubes of which have a particularly fixed bias of different values.

In brief, a common cyclic varying frequency oscillator such as disclosed in my copending application Ser. No. 505,530, filed December 30, 1930, provides a cyclic varying frequency. This output is passed through a reactance and rectifier, producing a uni-directional voltage of a value proportional to the frequency generated. This voltage is applied to the biasing circuits of the vacuum tubes. Of course, a voltage having like characteristics may be obtained directly from a constant frequency generator without the preliminary generation of a cyclic frequency current. As the bias is varied, each valve unit becomes operative for a certain unit of time, depending upon the cyclic variation. The signals will be transmitted during these time elements, transmission occurring at mutually exclusive intervals in a serial order to produce the transmission of the particular signals desired. Synchronization is obtained between the transmission and receiving oscillators. to provide the proper connections of the receiving circuits with the transmission channels, and isolate and separate each circuit on the common line.

The invention will be more fully understood by reference to the following description in conjunction with the accompanying drawings, in which:

Figure 1 shows a transmitting circuit in accordance with the invention.

Figs. 2 and 3 show diagrams of voltage-irequency characteristics of the different channels.

Fig. 4 shows a receiving system for the transmitter of Fig. 1; and

Fig. 5 is a schematic circuit of a variable cyclic frequency oscillator.

Referring to Fig. 1, three channels A, B and C are shown, having microphone sources 5 and 6 for channels A and B, respectively, and a photoelectric cell 7 associated with a television system not shown, forthe source of signals of channel C. These three signals may be locally amplified by amplifiers 9, 10 and 11, respectively, and after transmission through coupling transformers 13, 14 and 15 are transmitted through the valve control units 17, 18 and 19, respectively. After transmission through the last mentioned elements, the signals are impressed upon a common output circuit 20 connected to a transmitter 21 for transmission over an antenna system 22 or wire circuit 23.

The thermionic valve control circuits comprise vacuum tubes 26, 27 and 28 with fixed grid potentials 30, 31 and 32, respectively, of increasing values; The grid biasing circuits are supplemented either in an additive or negative sense by current fiow in resistances 34, 35 and 36 of substantially the same ohmic resistance. Two means of supplying a variable potential to the resistances 34, 35 and 36 are shown. One is a cyclic variable frequency oscillator 37, the circuit of which is shown in Fig. 5, and the usual type of voltage oscillator 41 of one definite frequency. The oscillator 37 develops a plurality of frequencies in a cyclic order, transmission through the reactance circuit 38 producing therefrom a voltage proportional to frequency. The alternating current is rectified in a rectifier 39 to obtain a uni-directional biasing voltage. The reactance circuit 38 is so constructed as to produce a voltage which is linearly related to frequency, in order to obtain a more uniform variation in the time operation of the valve controlled units; but this relationship is not required for successful operation of the circuit. If a single oscillator 41 is used, this oscillator may be connected in the circuit, as shown by the dotted conductors. A pair of conductors 40 connect the oscillators 3'7 and 41 with the transmitter 21 for the transmission of current impulses for the synchronism of the receiving and sending oscillators. The methodof synchronizing these circuits is explained in more detail in my copending application Ser. No. 510,860, filed January 24, 1931.

Referring to Figs. 2 and 3, an idea of the variation of the valve circuits may be obtained. The curve a shows the relationship between frequency and voltage at the output of the reactance unit 38. This curve is produced by the varying frequency output of the oscillator 37 present at the input of the reactor 38, shown by curve b which has time as its vertical coordinate. The vertical lines show the channels A, B and C plotted against the voltage characteristic curve from the output of the reactor 38, as well as the frequency characteristic curve from the oscillator 37.

In Fig. 3 the method of separating the signals by employing the well known characteristics of thermionic vacuum tubes is shown. This composite graph shows the characteristic curves of the tubes, these tubes being displaced with respect to the vertical coordinate, the horizontal coordinate representing the total voltage output from the oscillators which is impressed upon all of the tubes. The horizontal lines at the junctions of the individual current shift the vertical coordinate so that each begins at zero plate current. For instance, the signal A in reaching the control unit 17 will be transmitted as long as the bias of the tube is not less than zero or over the value shown at point 0. At the point 0 the bias is so great that the tube is rendered inoperative by reaching a saturation point on the EgIp curve. This operation characteristic is fixed by the grid bias 30 on the vacuum tube 26. The channel B is controlled by the fixed grid bias 31 which permits the tube 2'7 to operate at the point where the tube 26 is made inoperative, so that this tube operates with a grid bias of from 0 positive to at positive. Similarly the channel C is controlled by the tube 28 with its largest fixed bias 32, this tube operating with biaspotentials between the values of d positive and 6 positive. That is, the simultaneous arrival of signals at the input of the three control channels will be transmitted therethrough ina serial order at mutually exclusive time intervals. To prevent interaction of the signals between the various circuits, that is, a signal in an inoperative channel being present in an operative channel, chokes 42 and 43 are employed having a low resistance with a comparatively high inductance. The signals are thus transmitted into the output circuit 20 at mutually exclusive intervals in accordance with the wave form of biasing potential on the control tubes.

The circuit of the oscillator for producing a variable cyclic frequency, is shown briefly in Fig. 5 as comprising an arcrail system 60 having rails 61, field winding 62 with its energy supply 63 and control therefor 64, and av starting gap 65. The vacuum tube 67 with its coupling capacity 68. grid inductance 69, variable capacitance 70 and fixed tuning inductance '71 as the primary of the output transformer, comprises the remainder of the oscillator. This oscillator is similar to the one disclosed in my copending application mentioned above, namely Ser. No.i505,530, filed December 30, 1930.

The receiving system shown in Fig. 4 for the above described transmitter includes a receiving antenna '72 or wire line '73 with receiving equipment 74 having a common output circuit 75 and a synchronizing circuit '76. An oscillator '7'? similar to oscillator 3'7 at the transmitter, the circuit of which is shown in Fig. 5, produces the timing voltage through the reactance circuit '78 and rectifier '79 to thermionic control units 80, 81 and 82. An oscillator 86 similar to the oscillator 41 at the transmitter may be connected into the circuit, as shown by the dotted conductors. The control of the units 80, 81 and 82 is by a plurality of interconnected resistances 33, 84 and 85 which produce on the respective tubes voltages which make the tubes operative at mutually exclusive time intervals. That is, the channel for signal A is operative during the time interval that vacuum tube 88 is operative, to permit the transmission of the signal from transformer 87 through the transformer 89 to a sound responsive device 90. During the next time interval the tube 88 ,is blocked and tube 91 of control unit 81 passes the B signal from transformer .92 through transformer 93 to a sound responsive device 94. Similarly, the television signal coming in over transformer 95 is transmitted through tube 96 during the next interval when that tube has been biased to the operative point, thesignal passing through transformer 97 to the televisor 98. During this time interval, tubes 88 and 91 are blocked. The control circuits 80, .81 and 82 have diiferent fixed biases 100, 101 and 102, respectively. These biases are proportioned similarly to the biasing potentials 30, 31 and 32 of the transmitter in Fig. 1.

It will be noted in the above multiplex transmission system that no filters are required for segregating various signals, the signals being divided on a time basis. The portionof any one signal eliminated is not detrimental to the continuity of .transmission, because of the rapid division of theoperative periods. The principle on whichcontinuity may be maintained by eliminating a largeproportion of the unit of transmission is described in my first above-mentioned copending application. In connection with this principle, it might be stated here that it has been found that if the period of transmission of a signal occurs at a sufficiently rapid frequency even though it is a small proportion of the actual signal, nevertheless continuity is obtainable by the rapid occurrence of the transmitted unit.

The full scope of the invention is defined by the appended claims.

What is claimed is:

1. In an electrical transmission circuit, means for generating a plurality of signals to be transmitted, individual circuits for each of said signals, control valves in each of said circuits having different biases, a cyclically variable frequenoy oscillator, and means for producing the same bias on each of said valves at mutually exclusive intervals.

2. In a multiplex transmission system, means for generating a plurality of signals to be transmitted in the same direction, a channel for each of said signals, a common output circuit for said channels, a control unit in each of said channels, eachof said control units including a vacuum tube differently biased from the tubes in the other units, a cyclically variablefrequency oscillator and means'utilizing the output of said oscillator for producing the same bias on each of said tubes in a serial order for correspondingly operating said channels.

3. In an electrical transmission circuit, means for generating a plurality of signals to be transmitted over common transmission means, individual channels for each of said signals in the input of said transmission means, control units in each of said individual channels, a cyclically variable frequency oscillator, and means for impressing the output of said oscillator on said control units, said oscillator output varying the transmissability of said control units in accordance with variations of frequency in the output of said oscillator.

4. In a signal transmission system, means for generating a plurality of signals to be transmitted over common transmission apparatus, individual circuits in which said signals are created, valve control units in each of said circuits, means for producing a fixed bias on each of said valves of different values, means for obtaining a current having a continuously varying frequency, means for obtaining cyclic voltage variations proportional to the frequencies of said current, means for producing the same bias on said valves from said voltage wave at mutually exclusive intervals for causing said circuits to be operative at mutually exclusive intervals, and means for combining the output of said circuits for transmission over said common transmission apparatus.

5. In a system for the transmission of a plurality of signals over a common transmission medium, a plurality of circuits in which different signals are generated simultaneously, vacuum tube control units in each of said circuits, a cyclically variable frequency oscillator producing a biasing voltage proportional to frequency and common to all of said control vacuum tubes, a common circuit interconnecting said signal circuits, means in said common circuit to prevent interference between said signals, means common to said oscillator output circuit for producing biasing voltages on said tubes simultaneously, and means for combining said signals in a common output circuit. I

6. In a signal transmission system, means for generating a plurality of signals to be transmitted, an individual circuit for each of said signals, a common circuit for all of said signals, control units in each of said individual circuits having fixed operating characteristics, means for generating a current having a continuously varying frequency, means for obtaining a voltage from said current proportional to the frequencies thereof, and means for providing each control unit with the same operating characteristic at mutually exclusive time intervals from said voltage.

7. A signal transmission system in accordance with claim 6, in which said last mentioned means includes serially arranged filters between said individual circuits for preventing interference between channels.

8. In a multiplex transmission system, means for generating three signals to be simultaneously transmitted in the same direction, individual circuits for each of said signals, control valves in each of said circuits having progressively fixed biases, means for generating a current having a continuously varying frequency, and means interconnecting said last mentioned means and said valves for producing the same bias on each of said valves at mutually exclusive portions of the output wave from said oscillator.

ALEXANDER MCLEAN NICOLSON. 

